Isolation and characterization of a novel IgD-binding protein from Moraxella catarrhalis

Establishment of a method to detect human immunoglobulin G1 binding microorganisms

In this study, we developed an original and rapid anti-idiotypic antibody-based ELISA method, different from the techniques defined in the literature for this purpose so far, to detect immunoglobulin binding proteins (IBP) on the surface of bacteria. The test antibody used in our study to detect IBP is a recombinant human immunoglobulin G1 Kappa molecule, and has been used as a drug, Tocilizumab (Actemra®), in humans for therapeutic purposes. As a result, the test antibody in the supernatant after centrifugation is reduced compared to the initial moment due to antibody binding. Staphylococcus aureus cowan 1 strain used as positive control causes at least a 50% decrease in OD value in this respect. A similar observation at this level indicated that among a total of 189 microorganisms tested, 3 Staphylococcus aureus and 1 MRSA carrying high-affinity IgBP showed greater than 50% inhibition. This level of inhibition was not detected in the remaining microorganisms.

Teleost IgD+IgM- B cells in gills and skin have a plasmablast profile, but functionally and phenotypically differ from IgM+IgD- B cells in these sites

Although most B cells in teleost systemic compartments co-express IgM and IgD on the surface, cells exclusively expressing either of the two Igs are common in fish mucosal tissues, providing us with a unique opportunity to further characterize IgD+IgM- B cells, an intriguing B cell subset. Hence, we compared the phenotype of IgD+IgM- cells to that of IgM+IgD- B cells in rainbow trout gills and skin, also establishing the response of these subsets to immune stimulation. The transcriptional profile and secreting capacity of IgD+IgM- B cells corresponded to that of cells that have started a differentiation program toward plasmablasts, similarly to IgM+IgD- B cells. Yet, IgM+IgD- B cells retained high levels of surface MHC II and antigen-processing abilities, while these were much lower in IgD+IgM- cells, suggesting important differences in their antigen-presenting capacities. Our findings contribute to a deeper understanding of the enigmatic role of IgD in mucosal surfaces.

Defects in mucosal immunity and nasopharyngeal dysbiosis in HSC-transplanted SCID patients with IL2RG/JAK3 deficiency

Both innate and adaptive lymphocytes have critical roles in mucosal defense that contain commensal microbial communities and protect against pathogen invasion. Here we characterize mucosal immunity in patients with severe combined immunodeficiency (SCID) receiving hematopoietic stem cell transplantation (HSCT) with or without myeloablation. We confirmed that pretransplant conditioning had an impact on innate (natural killer and innate lymphoid cells) and adaptive (B and T cells) lymphocyte reconstitution in these patients with SCID and now show that this further extends to generation of T helper 2 and type 2 cytotoxic T cells. Using an integrated approach to assess nasopharyngeal immunity, we identified a local mucosal defect in type 2 cytokines, mucus production, and a selective local immunoglobulin A (IgA) deficiency in HSCT-treated SCID patients with genetic defects in IL2RG/GC or JAK3. These patients have a reduction in IgA-coated nasopharyngeal bacteria and exhibit microbial dysbiosis with increased pathobiont carriage. Interestingly, intravenous immunoglobulin replacement therapy can partially normalize nasopharyngeal immunoglobulin profiles and restore microbial communities in GC/JAK3 patients. Together, our results suggest a potential nonredundant role for type 2 immunity and/or of local IgA antibody production in the maintenance of nasopharyngeal microbial homeostasis and mucosal barrier function.

Rad52 mediates class-switch DNA recombination to IgD

In B cells, IgD is expressed together with IgM through alternative splicing of primary VHDJH-Cμ-s-m-Cδ-s-m RNAs, and also through IgD class switch DNA recombination (CSR) via double-strand DNA breaks (DSB) and synapse of Sμ with σδ. How such DSBs are resolved is still unknown, despite our previous report showing that Rad52 effects the ‘short-range’ microhomology-mediated synapsis of intra-Sμ region DSBs. Here we find that induction of IgD CSR downregulates Zfp318, and promotes Rad52 phosphorylation and recruitment to Sμ and σδ, thereby leading to alternative end-joining (A-EJ)-mediated Sμ-σδ recombination with extensive microhomologies, VHDJH-Cδs transcription and sustained IgD secretion. Rad52 ablation in mouse Rad52−/− B cells aborts IgD CSR in vitro and in vivo and dampens the specific IgD antibody response to OVA. Rad52 knockdown in human B cells also abrogates IgD CSR. Finally, Rad52 phosphorylation is associated with high levels of IgD CSR and anti-nuclear IgD autoantibodies in patients with systemic lupus erythematosus and in lupus-prone mice. Our findings thus show that Rad52 mediates IgD CSR through microhomology-mediated A-EJ in concert with Zfp318 downregulation.

Traditional Medicinal Plants—A Possible Source of Antibacterial Activity on Respiratory Diseases Induced by Chlamydia pneumoniae, Haemophilus influenzae, Klebsiella pneumoniae and Moraxella catarrhalis

Background. Nowadays, phytotherapy offers viable solutions in managing respiratory infections, disorders known for considerable incidence in both children and adults. In a context in which more and more people are turning to phytotherapy, finding new remedies is a topical goal of researchers in health and related fields. This paper aims to identify those traditional medicinal plants that show potentially antibacterial effects against four Gram-negative germs (Chlamydia pneumoniae, Haemophilus influenzae, Klebsiella pneumoniae, and Moraxella catarrhalis), which are considered to have high involvement in respiratory infections. Furthermore, a comparison with Romanian folk medicines was performed. Methods. An extensive review of books and databases was undertaken to identify vegetal species of interest in the context of the topic. Results. Some traditional Romanian species (such as Mentha × piperita, Thymus vulgaris, Pinus sylvestris, Allium sativum, Allium cepa, Ocimum basilicum, and Lavandulaangustifolia) were identified and compared with the plants and preparations confirmed as having antibacterial effects against specific germs. Conclusions. The antibacterial effects of some traditionally used Romanian medicinal plants are poorly investigated, and deserve further attention.

Antibacterial Fusion Proteins Enhance Moraxella catarrhalis Killing

Moraxella catarrhalis is a human-specific commensal of the respiratory tract and an opportunistic pathogen. It is one of the leading cause of otitis media in children and of acute exacerbations in patients with chronic obstructive pulmonary disease, resulting in significant morbidity and economic burden. Vaccines and new immunotherapeutic strategies to treat this emerging pathogen are needed. Complement is a key component of innate immunity that mediates the detection, response, and subsequent elimination of invading pathogens. Many pathogens including M. catarrhalis have evolved complement evasion mechanisms, which include the binding of human complement inhibitors such as C4b-binding protein (C4BP) and Factor H (FH). Inhibiting C4BP and FH acquisition by M. catarrhalis may provide a novel therapeutic avenue to treat infections. To achieve this, we created two chimeric proteins that combined the Moraxella-binding domains of C4BP and FH fused to human immunoglobulin Fcs: C4BP domains 1 and 2 and FH domains 6 and 7 fused to IgM and IgG Fc, respectively. As expected, FH6-7/IgG displaced FH from the bacterial surface while simultaneously activating complement via Fc-C1q interactions, together increasing pathogen elimination. C4BP1-2/IgM also increased serum killing of the bacteria through enhanced complement deposition, but did not displace C4BP from the surface of M. catarrhalis. These Fc fusion proteins could act as anti-infective immunotherapies. Many microbes bind the complement inhibitors C4BP and FH through the same domains as M. catarrhalis, therefore these Fc fusion proteins may be promising candidates as adjunctive therapy against many different drug-resistant pathogens.

Persistence of Moraxella catarrhalis in Chronic Obstructive Pulmonary Disease and Regulation of the Hag/MID Adhesin

BACKGROUND

Persistence of bacterial pathogens in the airways has profound consequences on the course and pathogenesis of chronic obstructive pulmonary disease (COPD). Patients with COPD continuously acquire and clear strains of Moraxella catarrhalis, a major pathogen in COPD. Some strains are cleared quickly and some persist for months to years. The mechanism of the variability in duration of persistence is unknown.

METHODS

Guided by genome sequences of selected strains, we studied the expression of Hag/MID, hag/mid gene sequences, adherence to human cells, and autoaggregation in longitudinally collected strains of M. catarrhalis from adults with COPD.

RESULTS

Twenty-eight of 30 cleared strains of M. catarrhalis expressed Hag/MID whereas 17 of 30 persistent strains expressed Hag/MID upon acquisition by patients. All persistent strains ceased expression of Hag/MID during persistence. Expression of Hag/MID in human airways was regulated by slipped-strand mispairing. Virulence-associated phenotypes (adherence to human respiratory epithelial cells and autoaggregation) paralleled Hag/MID expression in airway isolates.

CONCLUSIONS

Most strains of M. catarrhalis express Hag/MID upon acquisition by adults with COPD and all persistent strains shut off expression during persistence. These observations suggest that Hag/MID is important for initial colonization by M. catarrhalis and that cessation of expression facilitates persistence in COPD airways.

Translating Recent Microbiome Insights in Otitis Media into Probiotic Strategies

The microbiota of the upper respiratory tract (URT) protects the host from bacterial pathogenic colonization by competing for adherence to epithelial cells and by immune response regulation that includes the activation of antimicrobial and (anti-)inflammatory components. However, environmental or host factors can modify the microbiota to an unstable community that predisposes the host to infection or inflammation. One of the URT diseases most often encountered in children is otitis media (OM). The role of pathogenic bacteria like Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis in the pathogenesis of OM is well documented. Results from next-generation-sequencing (NGS) studies reveal other bacterial taxa involved in OM, such as Turicella and Alloiococcus Such studies can also identify bacterial taxa that are potentially protective against URT infections, whose beneficial action needs to be substantiated in relevant experimental models and clinical trials. Of note, lactic acid bacteria (LAB) are members of the URT microbiota and associated with a URT ecosystem that is deemed healthy, based on NGS and some experimental and clinical studies. These observations have formed the basis of this review, in which we describe the current knowledge of the molecular and clinical potential of LAB in the URT, which is currently underexplored in microbiome and probiotic research.

Total serum IgD from healthy and sick dogs with leishmaniosis

Background

Canine leishmaniosis (CanL) due to Leishmania infantum is characterized by the development of both cellular and humoral immune responses. The dysfunction of T cell-mediated immunity leads to a lack of proliferation of T cells in response to Leishmania antigens with the consequence of parasite dissemination that seems to be related to a T cell exhaustion mediated by regulatory B cells expressing immunoglobulin D (IgD). The aim of this study was to determine and compare the total serum IgD in dogs with clinical leishmaniosis and in clinically healthy dogs.

Results

A total of 147 dog sera were studied. All dogs were tested for L. infantum-specific antibodies by quantitative ELISA. Interferon-gamma (IFN-γ) production was also determined by sandwich ELISA after blood stimulation with L. infantum soluble antigen (LSA) or concanavalin A (ConA). The quantification of total IgD was performed using a human IgD sandwich ELISA quantification set. Dogs were classified in three different groups. Group 1 included 40 clinically healthy non-infected dogs, all serologically negative to L. infantum-specific antibodies and non-producers of IFN-γ upon LSA stimulation. Group 2 included 63 clinically healthy infected dogs that were LSA IFN-γ producers (n = 61) and/or IFN-γ non-producers (n = 2) as well as negative to medium seropositive to L. infantum antigen. Finally, Group 3 included 44 dogs with clinical leishmaniosis (IFN-γ producers, n = 23; and IFN-γ non-producers, n = 21) that were negative to highly positive to L. infantum-specific antibodies. No significant differences were observed when the total IgD concentration was compared within groups. Additionally, total IgD of sick IFN-γ producers and IFN-γ non-producers was not significantly different. Finally, total IgD concentration was not statistically related to demographic parameters such as age, sex and breed.

Conclusions

The results of this study demonstrated that there were no differences between groups in total serum IgD. Total serum IgD does not appear to be a marker of disease in CanL.

Short Leucine-Rich Proteoglycans Modulate Complement Activity and Increase Killing of the Respiratory Pathogen Moraxella catarrhalis

The respiratory pathogen Moraxella catarrhalis is a human-specific commensal that frequently causes acute otitis media in children and stimulates acute exacerbations in chronic obstructive pulmonary disease patients. The exact molecular mechanisms defining host-pathogen interactions promoting pathogenesis are not clearly understood. Limited knowledge hampers vaccine and immunotherapeutic development required to treat this emerging pathogen. In this study, we reveal in detail a novel antibacterial role displayed by short leucine-rich proteoglycans (SLRPs) in concert with complement. We show that fibromodulin (FMOD), osteoadherin (OSAD), and biglycan (BGN) but not decorin (DCN) enhance serum killing of M. catarrhalis. Our results suggest that M. catarrhalis binding to SLRPs is a conserved feature, as the overwhelming majority of clinical and laboratory strains bound all four SLRPs. Furthermore, we resolve the binding mechanism responsible for this interaction and highlight the role of the ubiquitous surface protein (Usp) A2/A2H in mediating binding to host SLRPs. A conserved immune evasive strategy used by M. catarrhalis and other pathogens is the surface acquisition of host complement inhibitors such as C4b-binding protein (C4BP). We observed that FMOD, OSAD, and BGN competitively inhibit binding of C4BP to the surface of M. catarrhalis, resulting in increased C3b/iC3b deposition, membrane attack complex (MAC) formation, and subsequently decreased bacterial survival. Furthermore, both OSAD and BGN promote enhanced neutrophil killing in vitro, both in a complement-dependent and independent fashion. In summary, our results illustrate that SLRPs, FMOD, OSAD, and BGN portray complement-modulating activity enhancing M. catarrhalis killing, defining a new antibacterial role supplied by SLRPs.

The enigmatic function of IgD: some answers at last

IgD emerged soon after IgM at the time of inception of the adaptive immune system. Despite its evolutionary conservation from fish to humans, the specific functions of IgD have only recently begun to be elucidated. Mature B cells undergo alternative mRNA splicing to express IgD and IgM receptors with identical antigenic specificity. The enigma of dual IgD and IgM expression has been tackled by several recent studies showing that IgD helps peripheral accumulation of physiologically autoreactive B cells through its functional unresponsiveness to self-antigens but prompt readiness against foreign antigens. IgD achieves this balance by attenuating IgM-mediated anergy while promoting specific responses to multimeric non-self-antigens. Additional research has clarified how and why certain mucosal B cells become plasmablasts or plasma cells specializing in IgD secretion. In particular, the microbiota has been shown to play an important role in driving class switch-mediated replacement of IgM with IgD. Secreted IgD appears to enhance mucosal homeostasis and immune surveillance by "arming" myeloid effector cells such as basophils and mast cells with IgD antibodies reactive against mucosal antigens, including commensal and pathogenic microbes. Here we will review these advances and discuss their implications in humoral immunity in human and mice.

Expression of immunoglobulin D is increased in chronic rhinosinusitis

BACKGROUND

Immunoglobulin (Ig) D is largely localized to the upper airway and reacts with colonizing respiratory pathogens.

OBJECTIVE

To determine whether chronic rhinosinusitis (CRS) is associated with increased IgD expression.

METHODS

We performed immunofluorescent staining for cytoplasmic IgD, IgA, IgM, and surface plasma cell marker CD138 (syndecan-1) in sinus tissue of patients with CRS with and without nasal polyps (CRSwNP and CRSsNP, respectively) and control subjects without CRS (n = 6 each). Sinonasal mucus antibody levels of patients with CRSwNP or CRSsNP and control subjects were measured by enzyme-linked immunosorbent assay (n = 13, 11, and 9 subjects, respectively). Cells per square millimeter and antibody levels were compared by analysis of variance. Histopathology was performed with sinus tissue from subjects in the 3 groups (n = 6, 8, and 13 subjects respectively).

RESULTS

Cells expressing cytoplasmic IgD exceeded those with cytoplasmic IgA and IgM and represented most CD138⁺ plasma cells in the lamina propria. The frequencies of IgD⁺ plasma cells were significantly higher in patients with CRSsNP and CRSwNP compared with control subjects (P < .01). Only patients with CRSwNP showed increased frequencies of IgM and IgA plasma cells (P < .01). In contrast to high plasma cell frequencies in tissues, the levels of secreted IgD were lower than those of IgA, IgM, and IgG but were highest in the CRSwNP group compared with the other groups (P < .05).

CONCLUSION

IgD plasma cells are prominent in sinus tissues and are increased in CRS. That IgD protein also shows the lowest concentration of antibodies in secretions suggests that its activity might be targeted to the tissue rather than secretions.

A Moraxella catarrhalis vaccine to protect against otitis media and exacerbations of COPD: An update on current progress and challenges

Moraxella catarrhalis is a major cause of morbidity and mortality worldwide, especially causing otitis media in young children and exacerbations of chronic obstructive pulmonary disease in adults. This pathogen uses several virulence mechanisms to colonize and survive in its host, including adherence and invasion of host cells, formation of polymicrobial biofilms with other bacterial pathogens, and production of β-lactamase. Given the global impact of otitis media and COPD, an effective vaccine to prevent M. catarrhalis infection would have a huge impact on the quality of life in both patient populations by preventing disease, thus reducing morbidity and health care costs. A number of promising vaccine antigens have been identified for M. catarrhalis. The development of improved animal models of M. catarrhalis disease and identification of a correlate of protection are needed to accelerate vaccine development. This review will discuss the current state of M. catarrhalis vaccine development, and the challenges that must be addressed to succeed.

Potential impact of a Moraxella catarrhalis vaccine in COPD

Moraxella catarrhalis is the second most common cause of exacerbations in adults with COPD, resulting in enormous morbidity and mortality in this clinical setting. Vaccine development for M. catarrhalis has lagged behind the other two important causes of exacerbations in COPD, nontypeable Haemophilus influenzae and Streptococcus pneumoniae. While no licensed vaccine is currently available for M. catarrhalis, several promising candidate vaccine antigens have been identified and characterized and are close to entering clinical trials. Key steps that are required to advance vaccines for M. catarrhalis along the translational pipeline include standardization of assay systems to assess candidate antigens, identification of a reliable correlate of protection and expansion of partnerships between industry, academia and government to overcome regulatory hurdles. A vaccine to prevent M. catarrhalis infections in COPD would have a major impact in reducing morbidity, mortality and healthcare costs in COPD.

Vaccine targets against Moraxella catarrhalis

INTRODUCTION

Moraxella catarrhalis is a prominent pathogen that causes acute otitis media in children and lower respiratory tract infections in adults, resulting in a significant socioeconomic burden on healthcare systems globally. No vaccine is currently available for M. catarrhalis. Promising M. catarrhalis target antigens have been characterized in animal models and should soon enter human clinical trials.

AREAS COVERED

This review discusses the detailed features and research status of current candidate target antigens for an M. catarrhalis vaccine. The approaches for assessing M. catarrhalis vaccine efficacy are also discussed.

EXPERT OPINION

Targeting the key molecules contributing to serum resistance may be a viable strategy to identify effective vaccine targets among M. catarrhalis antigens. Elucidating the role and mechanisms of the serum and mucosal immune responses to M. catarrhalis is significant for vaccine target selection, testing and evaluation. Developing animal models closely simulating M. catarrhalis-caused human respiratory diseases is of great benefit in better understanding pathogenesis and evaluating vaccine efficacy. Carrying out clinical trials will be a landmark in the progress of M. catarrhalis vaccine research. Combined multicomponent vaccines will be a focus of future M. catarrhalis vaccine studies.

A fusion protein derived from Moraxella catarrhalis and Neisseria meningitidis aimed for immune modulation of human B cells

Moraxella IgD-binding protein (MID) is a well characterized trimeric autotransporter that specifically targets the IgD of B cells. We fused the membrane anchor of the meningococcal autotransporter NhhA with the IgD-binding region of MID (aa 962-1200) to create a chimeric protein designated as NID. The aim was to use this specific targeting to provide a better vaccine candidate against meningococci, in particular serogroup B by enhancing the immunogenicity of NhhA. NID was thereafter recombinantly expressed in E. coli. The NID-expressing E. coli bound to peripheral B lymphocytes that resulted in cellular activation. Furthermore, we also successfully expressed NID on outer membrane vesicles, nanoparticles that are commonly used in meningococcal vaccines. This study thus highlights the applicability of the menigococcal-Moraxella fusion protein NID to be used for specific targeting of vaccine components to the IgD B cell receptor.

Binding of Streptococcus pneumoniae endopeptidase O (PepO) to complement component C1q modulates the complement attack and promotes host cell adherence

The Gram-positive species Streptococcus pneumoniae is a human pathogen causing severe local and life-threatening invasive diseases associated with high mortality rates and death. We demonstrated recently that pneumococcal endopeptidase O (PepO) is a ubiquitously expressed, multifunctional plasminogen and fibronectin-binding protein facilitating host cell invasion and evasion of innate immunity. In this study, we found that PepO interacts directly with the complement C1q protein, thereby attenuating the classical complement pathway and facilitating pneumococcal complement escape. PepO binds both free C1q and C1 complex in a dose-dependent manner based on ionic interactions. Our results indicate that recombinant PepO specifically inhibits the classical pathway of complement activation in both hemolytic and complement deposition assays. This inhibition is due to direct interaction of PepO with C1q, leading to a strong activation of the classical complement pathway, and results in consumption of complement components. In addition, PepO binds the classical complement pathway inhibitor C4BP, thereby regulating downstream complement activation. Importantly, pneumococcal surface-exposed PepO-C1q interaction mediates bacterial adherence to host epithelial cells. Taken together, PepO facilitates C1q-mediated bacterial adherence, whereas its localized release consumes complement as a result of its activation following binding of C1q, thus representing an additional mechanism of human complement escape by this versatile pathogen.

Haemophilus influenzae resides in tonsils and uses immunoglobulin D binding as an evasion strategy

Haemophilus influenzae (Hi) causes respiratory tract infections and is also considered to be a commensal, particularly in preschool children. Tonsils from patients (n = 617) undergoing tonsillectomy due to chronic infection or hypertrophy were examined. We found that 51% of tonsils were positive for Hi, and in 95% of cases analyzed in detail (n = 39) Hi resided intracellularly in the core tonsillar tissue. Patients harbored several intracellular unique strains and the majority were nontypeable Hi (NTHi). Interestingly, the isolated NTHi bound soluble immunoglobulin (Ig) D at the constant heavy chain domain 1 as revealed by recombinant IgD/IgG chimeras. NTHi also interacted with B lymphocytes via the IgD B-cell receptor, resulting in internalization of bacteria, T-cell-independent activation via Toll-like receptor 9, and differentiation into non-NTHi-specific IgM-producing cells. Taken together, IgD-binding NTHi leads to an unspecific immune response and may support the bacteria to circumvent the host defense.

Use of the chinchilla model for nasopharyngeal colonization to study gene expression by Moraxella catarrhalis

Young adult chinchillas were atraumatically inoculated with Moraxella catarrhalis via the nasal route. Detailed histopathologic examination of nasopharyngeal tissues isolated from these M. catarrhalis-infected animals revealed the presence of significant inflammation within the epithelium. Absence of similar histopathologic findings in sham-inoculated animals confirmed that M. catarrhalis was exposed to significant host-derived factors in this environment. Twenty-four hours after inoculation, viable M. catarrhalis organisms were recovered from the nasal cavity and nasopharynx of the animals in numbers sufficient for DNA microarray analysis. More than 100 M. catarrhalis genes were upregulated in vivo, including open reading frames (ORFs) encoding proteins that are involved in a truncated denitrification pathway or in the oxidative stress response, as well as several putative transcriptional regulators. Additionally, 200 M. catarrhalis genes were found to be downregulated when this bacterium was introduced into the nasopharynx. These downregulated genes included ORFs encoding several well-characterized M. catarrhalis surface proteins including Hag, McaP, and MchA1. Real-time reverse transcriptase PCR (RT-PCR) was utilized as a stringent control to validate the results of in vivo gene expression patterns as measured by DNA microarray analysis. Inactivation of one of the genes (MC ORF 1550) that was upregulated in vivo resulted in a decrease in the ability of M. catarrhalis to survive in the chinchilla nasopharynx over a 3-day period. This is the first evaluation of global transcriptome expression by M. catarrhalis cells in vivo.

The function and regulation of immunoglobulin D

Recent discoveries of IgD in ancient vertebrates suggest that IgD has been preserved in evolution from fish to human for important immunological functions. A non-canonical form of class switching from IgM to IgD occurs in the human upper respiratory mucosa to generate IgD-secreting B cells that bind respiratory bacteria and their products. In addition to enhancing mucosal immunity, IgD class-switched B cells enter the circulation to 'arm' basophils and other innate immune cells with secreted IgD. Although the nature of the IgD receptor remains elusive, cross-linking of IgD on basophils stimulates release of immunoactivating, proinflammatory and antimicrobial mediators. This pathway is dysregulated in autoinflammatory disorders such as hyper-IgD syndrome, indicating that IgD orchestrates an ancestral surveillance system at the interface between immunity and inflammation.

Immune evasion of Moraxella catarrhalis involves ubiquitous surface protein A-dependent C3d binding

The complement system plays an important role in eliminating invading pathogens. Activation of complement results in C3b deposition (opsonization), phagocytosis, anaphylatoxin (C3a, C5a) release, and consequently cell lysis. Moraxella catarrhalis is a human respiratory pathogen commonly found in children with otitis media and in adults with chronic obstructive pulmonary disease. The species has evolved multiple complement evasion strategies, which among others involves the ubiquitous surface protein (Usp) family consisting of UspA1, A2, and A2 hybrid. In the present study, we found that the ability of M. catarrhalis to bind C3 correlated with UspA expression and that C3 binding contributed to serum resistance in a large number of clinical isolates. Recombinantly expressed UspA1 and A2 inhibit both the alternative and classical pathways, C3b deposition, and C3a generation when bound to the C3 molecule. We also revealed that the M. catarrhalis UspA-binding domain on C3b was located to C3d and that the major bacterial C3d-binding domains were within UspA1(299-452) and UspA2(165-318). The interaction with C3 was not species specific since UspA-expressing M. catarrhalis also bound mouse C3 that resulted in inhibition of the alternative pathway of mouse complement. Taken together, the binding of C3 to UspAs is an efficient strategy of Moraxella to block the activation of complement and to inhibit C3a-mediated inflammation.

New insights into the enigma of immunoglobulin D

Immunoglobulin D (IgD) has remained a mysterious antibody class for almost half a century. IgD was initially thought to be a recently evolved Ig isotype expressed only by some mammalian species, but recent discoveries in fishes and amphibians demonstrate that IgD was present in the ancestor of all jawed vertebrates and has important immunological functions. The structure of IgD has been very dynamic throughout evolution. Mammals can express IgD through alternative splicing and class switch recombination. Active cell-dependent and T-cell-independent IgM-to-IgD class switching takes place in a unique subset of human B cells from the upper aerodigestive mucosa, which provides a layer of mucosal protection by interacting with many pathogens and their virulence factors. Circulating IgD can bind to myeloid cells such as basophils and induce antimicrobial, inflammatory, and B-cell-stimulating factors upon cross-linking, which contributes to not only immune surveillance but also inflammation and tissue damage when this pathway is overactivated under pathological conditions. Recent research shows that IgD is an important immunomodulator that orchestrates an ancestral surveillance system at the interface between immunity and inflammation.

Colonization of healthy children by Moraxella catarrhalis is characterized by genotype heterogeneity, virulence gene diversity and co-colonization with Haemophilus influenzae

The colonization dynamics of Moraxella catarrhalis were studied in a population comprising 1079 healthy children living in Rotterdam, The Netherlands (the Generation R Focus cohort). A total of 2751 nasal swabs were obtained during four clinic visits timed to take place at 1.5, 6, 14 and 24 months of age, yielding a total of 709 M. catarrhalis and 621 Haemophilus influenzae isolates. Between January 2004 and December 2006, approximate but regular 6-monthly cycles of colonization were observed, with peak colonization incidences occurring in the autumn/winter for M. catarrhalis, and winter/spring for H. influenzae. Co-colonization was significantly more likely than single-species colonization with either M. catarrhalis or H. influenzae, with genotypic analysis revealing no clonality for co-colonizing or single colonizers of either bacterial species. This finding is especially relevant considering the recent discovery of the importance of H. influenzae-M. catarrhalis quorum sensing in biofilm formation and host clearance. Bacterial genotype heterogeneity was maintained over the 3-year period of the study, even within this relatively localized geographical region, and there was no association of genotypes with either season or year of isolation. Furthermore, chronological and genotypic diversity in three immunologically important M. catarrhalis virulence genes (uspA1, uspA2 and hag/mid) was also observed. This study indicates that genotypic variation is a key factor contributing to the success of M. catarrhalis colonization of healthy children in the first years of life. Furthermore, variation in immunologically relevant virulence genes within colonizing populations, and even within genotypically identical M. catarrhalis isolates, may be a result of immune evasion by this pathogen. Finally, the factors facilitating M. catarrhalis and H. influenzae co-colonization need to be further investigated.

Identification of two IgD+ B cell populations in channel catfish, Ictalurus punctatus

Channel catfish Ictalurus punctatus express two Ig isotypes: IgM and IgD. Although catfish IgM has been extensively studied at the functional and structural levels, much less is known about IgD. In this study, IgM(+)/IgD(+) and IgM(-)/IgD(+) catfish B cell populations were identified through the use of anti-IgM and anti-IgD mAbs. Catfish IgM(+)/IgD(+) B cells are small and agranular. In contrast, IgM(-)/IgD(+) B cells are larger and exhibit a plasmablast morphology. The use of cell sorting, flow cytometry, and RT-PCR demonstrated that IgD(+) B cell expression varies among individuals. For example, some catfish have <5% IgM(-)/IgD(+) B cells in their PBLs, whereas in others the IgM(-)/IgD(+) B cell population can represent as much as 72%. Furthermore, IgD expressed by IgM(-)/IgD(+) B cells preferentially associates with IgL σ. Comparatively, IgM(+)/IgD(+) B cells can express any of the four catfish IgL isotypes. Also, transfection studies show that IgD functions as a typical BCR, because Igδ-chains associate with CD79a and CD79b molecules, and all membrane IgD transcripts from sorted IgM(-)/IgD(+) B cells contain viable VDJ rearrangements, with no bias in family member usage. Interestingly, all secreted IgD transcripts from IgM(+)/IgD(+) and IgM(-)/IgD(+) B cells were V-less and began with a leader spliced to Cδ1. Importantly, transfection of catfish clonal B cells demonstrated that this leader mediated IgD secretion. Together, these findings imply that catfish IgM(-)/IgD(+) B cells likely expand in response to certain pathogens and that the catfish IgD Fc-region, as has been suggested for human IgD, may function as a pattern recognition molecule.

B cell activation by outer membrane vesicles--a novel virulence mechanism

Secretion of outer membrane vesicles (OMV) is an intriguing phenomenon of Gram-negative bacteria and has been suggested to play a role as virulence factors. The respiratory pathogens Moraxella catarrhalis reside in tonsils adjacent to B cells, and we have previously shown that M. catarrhalis induce a T cell independent B cell response by the immunoglobulin (Ig) D-binding superantigen MID. Here we demonstrate that Moraxella are endocytosed and killed by human tonsillar B cells, whereas OMV have the potential to interact and activate B cells leading to bacterial rescue. The B cell response induced by OMV begins with IgD B cell receptor (BCR) clustering and Ca(2+) mobilization followed by BCR internalization. In addition to IgD BCR, TLR9 and TLR2 were found to colocalize in lipid raft motifs after exposure to OMV. Two components of the OMV, i.e., MID and unmethylated CpG-DNA motifs, were found to be critical for B cell activation. OMV containing MID bound to and activated tonsillar CD19(+) IgD(+) lymphocytes resulting in IL-6 and IgM production in addition to increased surface marker density (HLA-DR, CD45, CD64, and CD86), whereas MID-deficient OMV failed to induce B cell activation. DNA associated with OMV induced full B cell activation by signaling through TLR9. Importantly, this concept was verified in vivo, as OMV equipped with MID and DNA were found in a 9-year old patient suffering from Moraxella sinusitis. In conclusion, Moraxella avoid direct interaction with host B cells by redirecting the adaptive humoral immune response using its superantigen-bearing OMV as decoys.

Molecular aspects of Moraxella catarrhalis pathogenesis

In recent years, Moraxella catarrhalis has established its position as an important human mucosal pathogen, no longer being regarded as just a commensal bacterium. Further, current research in the field has led to a better understanding of the molecular mechanisms involved in M. catarrhalis pathogenesis, including mechanisms associated with cellular adherence, target cell invasion, modulation of the host's immune response, and metabolism. Additionally, in order to be successful in the host, M. catarrhalis has to be able to interact and compete with the commensal flora and overcome stressful environmental conditions, such as nutrient limitation. In this review, we provide a timely overview of the current understanding of the molecular mechanisms associated with M. catarrhalis virulence and pathogenesis.

Nontypeable Haemophilus influenzae protein E binds vitronectin and is important for serum resistance

Nontypeable Haemophilus influenzae (NTHi) commonly causes local disease in the upper and lower respiratory tract and has recently been shown to interfere with both the classical and alternative pathways of complement activation. The terminal pathway of the complement system is regulated by vitronectin that is a component of both plasma and the extracellular matrix. In this study, we identify protein E (PE; 16 kDa), which is a recently characterized ubiquitous outer membrane protein, as a vitronectin-binding protein of NTHi. A PE-deficient NTHi mutant had a markedly reduced survival in serum compared with the PE-expressing isogenic NTHi wild type. Moreover, the PE-deficient mutant showed a significantly decreased binding to both soluble and immobilized vitronectin. In parallel, PE-expressing Escherichia coli bound soluble vitronectin and adhered to immobilized vitronectin compared with controls. Surface plasmon resonance technology revealed a K(D) of 0.4 microM for the interaction between recombinant PE and immobilized vitronectin. Moreover, the PE-dependent vitronectin-binding site was located at the heparin-binding domains of vitronectin and the major vitronectin-binding domain was found in the central core of PE (aa 84-108). Importantly, vitronectin bound to the surface of NTHi 3655 reduced membrane attack complex-induced hemolysis. In contrast to incubation with normal human serum, NTHi 3655 showed a reduced survival in vitronectin-depleted human serum, thus demonstrating that vitronectin mediates a protective role at the bacterial surface. Our findings show that PE, by binding vitronectin, may play an important role in NTHi pathogenesis.

Superantigen- and TLR-dependent activation of tonsillar B cells after receptor-mediated endocytosis

Classical B lymphocyte activation is dependent on BCR cross-linking in combination with physical interaction with Th cells. Other B cell molecules that contribute to the activation are complement, cytokine, and TLRs recognizing specific pathogen-associated molecular patterns. Moraxella (Branhamella) catarrhalis is a common Gram-negative respiratory pathogen that induces proliferation in human IgD-expressing B cells independently of T cell help. The activation is initiated by the B cell superantigen Moraxella IgD-binding protein (MID) through a nonimmune cross-linking of IgD. However, IgD cross-linking alone is not sufficient to induce proliferation. In this study, we characterized the significance of TLRs in superantigen-dependent B cell activation using whole bacteria or rMID in the presence or absence of TLR ligands. IgD cross-linking by MID sensitized B cells obtained from children with tonsillar hyperplasia for mainly TLR9, whereas TLRs 1, 2, 6, and 7 were less important. The Moraxella-induced activation was inhibited when a dominant-negative TLR9 ligand was added. Interestingly, BCR-mediated endocytosis of whole Moraxella and degradation of live bacteria in naive B cells were observed with fluorescence, confocal, and transmission electron microscopy. This unique observation proved the strong intracellular TLR9 response as well as highlighted the Ag-presenting function of B cells. In conclusion, our findings suggest an important role of TLRs in the adaptive immune response and reveal novel insights into the T cell-independent B cell activation induced by bacteria.

Identification of domains of the Hag/MID surface protein recognized by systemic and mucosal antibodies in adults with chronic obstructive pulmonary disease following clearance of Moraxella catarrhalis

Moraxella catarrhalis is a common cause of respiratory tract infection in the setting of chronic obstructive pulmonary disease (COPD). Adults with COPD acquire and clear strains of M. catarrhalis from the respiratory tract continuously and develop strain-specific protection following clearance of a strain. In previous work, we identified Hag/MID (Moraxella immunoglobulin D-binding protein), a large multifunctional surface protein that acts as an adhesin and hemagglutinin, as a target of antibody responses in adults with COPD after clearance of M. catarrhalis. The goal of the present study was to characterize the domains of Hag/MID to which humans make antibodies, including both systemic and mucosal antibody responses. Analysis of recombinant peptide constructs, which spanned the M. catarrhalis strain O35E Hag/MID protein, with well-characterized serum and sputum samples revealed that most adults with COPD made antibodies directed toward a region of the molecule bounded by amino acids 706 to 863. Serum immunoglobulin G (IgG) and IgA purified from sputum both recognized the same domain. Some flanking sequence of this fragment was necessary for the epitope(s) in this region to maintain its conformation to bind human antibodies. These results reveal that humans consistently generate both systemic and mucosal antibody responses to an immunodominant region of the Hag/MID molecule, which was previously shown to overlap with several biologically relevant domains, including epithelial cell adherence, IgD binding, collagen binding, and hemagglutination.

Current progress of adhesins as vaccine candidates for Moraxella catarrhalis

Moraxella catarrhalis is an emerging pathogen and all isolates are now considered beta-lactamase producing. Potential further use of vaccines against Streptococcus pneumoniae and nontypeable Haemophilus influenzae means that M. catarrhalis might be thrust further into the limelight. However, a vaccine has not yet been designed. In this review, the progress of M. catarrhalis adhesins as vaccine candidates is discussed with a focus on various candidate antigens that spanned those discovered more than 10 years ago, for example, the ubiquitous surface proteins to newer antigens, such as the Moraxella IgD-binding hemagglutinin.

Moraxella catarrhalis synthesizes an autotransporter that is an acid phosphatase

Moraxella catarrhalis O35E was shown to synthesize a 105-kDa protein that has similarity to both acid phosphatases and autotransporters. The N-terminal portion of the M. catarrhalis acid phosphatase A (MapA) was most similar (the BLAST probability score was 10(-10)) to bacterial class A nonspecific acid phosphatases. The central region of the MapA protein had similarity to passenger domains of other autotransporter proteins, whereas the C-terminal portion of MapA resembled the translocation domain of conventional autotransporters. Cloning and expression of the M. catarrhalis mapA gene in Escherichia coli confirmed the presence of acid phosphatase activity in the MapA protein. The MapA protein was shown to be localized to the outer membrane of M. catarrhalis and was not detected either in the soluble cytoplasmic fraction from disrupted M. catarrhalis cells or in the spent culture supernatant fluid from M. catarrhalis. Use of the predicted MapA translocation domain in a fusion construct with the passenger domain from another predicted M. catarrhalis autotransporter confirmed the translocation ability of this MapA domain. Inactivation of the mapA gene in M. catarrhalis strain O35E reduced the acid phosphatase activity expressed by this organism, and this mutation could be complemented in trans with the wild-type mapA gene. Nucleotide sequence analysis of the mapA gene from six M. catarrhalis strains showed that this protein was highly conserved among strains of this pathogen. Site-directed mutagenesis of a critical histidine residue (H233A) in the predicted active site of the acid phosphatase domain in MapA eliminated acid phosphatase activity in the recombinant MapA protein. This is the first description of an autotransporter protein that expresses acid phosphatase activity.

Regions important for the adhesin activity of Moraxella catarrhalis Hag

BACKGROUND

The Moraxella catarrhalis Hag protein, an Oca autotransporter adhesin, has previously been shown to be important for adherence of this respiratory tract pathogen to human middle ear and A549 lung cells.

RESULTS

The present study demonstrates that adherence of M. catarrhalis isogenic hag mutant strains to the human epithelial cell lines Chang (conjunctival) and NCIH292 (lung) is reduced by 50-93%. Furthermore, expressing Hag in a heterologous Escherichia coli background substantially increased the adherence of recombinant bacteria to NCIH292 cells and murine type IV collagen. Hag did not, however, increase the attachment of E. coli to Chang cells. These results indicate that Hag directly mediates adherence to NCIH292 lung cells and collagen, but is not sufficient to confer binding to conjunctival monolayers. Several in-frame deletions were engineered within the hag gene of M. catarrhalis strain O35E and the resulting proteins were tested for their ability to mediate binding to NCIH292 monolayers, middle ear cells, and type IV collagen. These experiments revealed that epithelial cell and collagen binding properties are separable, and that residues 385-705 of this ~2,000 amino acid protein are important for adherence to middle ear and NCIH292 cells. The region of O35E-Hag encompassing aa 706 to 1194 was also found to be required for adherence to collagen. In contrast, beta-roll repeats present in Hag, which are structural features conserved in several Oca adhesins and responsible for the adhesive properties of Yersinia enterocolitica YadA, are not important for Hag-mediated adherence.

CONCLUSION

Hag is a major adherence factor for human cells derived from various anatomical sites relevant to pathogenesis by M. catarrhalis and its structure-function relationships differ from those of other, closely-related autotransporter proteins.

Characterization of the IgD binding site of encapsulated Haemophilus influenzae serotype b

Encapsulated Haemophilus influenzae is a causative agent of invasive disease, such as meningitis and septicemia. Several interactions exist between H. influenzae and the human host. H. influenzae has been reported to bind IgD in a nonimmune manner, but the responsible protein has not yet been identified. To define the binding site on IgD for H. influenzae, full-length IgD and four chimeric IgDs with interspersed IgG sequences and Ag specificity for dansyl chloride were expressed in stably transfected Chinese hamster ovary cells. The binding of recombinant IgD to a panel of encapsulated H. influenzae serotype b (Hib) and nontypeable strains were investigated using a whole cell ELISA and flow cytometry. IgD binding was detected in 50% of the encapsulated Hib strains examined, whereas nontypeable H. influenzae did not interact with IgD. Finally, mapping experiments using the chimeric IgD/IgG indicated that IgD CH1 aa 198-224 were involved in the interaction between IgD and H. influenzae. Thus, by using recombinant IgD and chimeras with defined Ag specificity, we have confirmed that Hib specifically binds IgD, and that this binding involves the IgD CH1 region.

Moraxella catarrhalis strain O35E expresses two filamentous hemagglutinin-like proteins that mediate adherence to human epithelial cells

Two-partner secretion (TPS) systems are a family of proteins being rapidly identified and characterized in a growing number of gram-negative bacteria. TPS systems mediate the secretion of proteins, many involved in virulence traits such as hemolysis, adherence to epithelial cells, inhibition of bacterial growth, and immunomodulation of the host. A TPS system typically consists of a transporter located in the bacterial outer membrane (OM) which is responsible for the recognition and secretion of at least one large exoprotein. Two of the better-characterized TPS systems specify the Bordetella pertussis FHA and Haemophilus influenzae HMW1/HMW2 proteins. We identified three gene products of Moraxella catarrhalis strain O35E that resemble TPS proteins and designated them MhaC (transporter), MhaB1 (exoprotein), and MhaB2 (exoprotein). Western blot analysis using anti-MhaC, or antibodies reacting to both MhaB1 and MhaB2 (MhaB-reactive), revealed that these antigens are expressed in the OM of 63% of isolates tested. Mutations in the mhaC gene specifying the putative transporter of the M. catarrhalis wild-type strains O35E, O12E, and McGHS1 resulted in the absence of MhaB1/MhaB2 in the OM of mutants. These results are therefore consistent with the Mha proteins functioning as a TPS system. Furthermore, we discovered that these mhaC mutants exhibit markedly decreased binding to human epithelial cells relevant to pathogenesis by M. catarrhalis (Chang, HEp2, A549, and/or 16HBE14o(-)). Expression of O12E MhaC and MhaB1 in a nonadherent strain of Escherichia coli was found to increase the adherence of recombinant bacteria to HEp2 monolayers by sevenfold, thereby demonstrating that this M. catarrhalis TPS system directly mediates binding to human epithelial cells. The construction of isogenic mutants in the mhaB1 and mhaB2 genes of strain O35E also suggests that the MhaB proteins play distinct roles in M. catarrhalis adherence.

A UspA2H-negative variant of Moraxella catarrhalis strain O46E has a deletion in a homopolymeric nucleotide repeat common to uspA2H genes

Moraxella catarrhalis strains can express either a UspA2 protein or a UspA2H protein. The latter protein is encoded by a gene that possesses a homopolymeric nucleotide tract containing eight adenine (A) residues [i.e., a poly(A) tract] which is located near the 5' end. A spontaneous UspA2H-negative variant of M. catarrhalis strain O46E, designated O46E.U2V, was found to have a uspA2H poly(A) tract that contained seven A residues. Northern blot analysis of total RNA from the O46E parent strain revealed a readily detectable uspA2H mRNA transcript, whereas little or no uspA2H transcript was detectable in total RNA from the UspA2H-negative variant O46E.U2V. The 5' end of the uspA2H genes from both the O46E parent strain and the O46E.U2V variant were ligated to a promoterless lacZ gene to prepare translational fusions for use as reporter constructs. The level of beta-galactosidase activity expressed by the fusion construct containing eight A residues in its poly(A) tract was 200-fold greater than that obtained with the construct that had seven A residues. Site-directed mutagenesis of the 5' end of the uspA2H gene confirmed that translation was initiated at a GTG codon located 21 nucleotides (nt) upstream of the poly(A) tract. Primer extension analysis determined that the transcriptional start site of the uspA2H gene was located 291 nt upstream from the GTG translational start codon. This poly(A) tract was also found to be present in the uspA2H genes of other M. catarrhalis strains.

Purification of IgD from human serum--a novel application of recombinant M. catarrhalis IgD-binding protein (MID)

Moraxella catarrhalis IgD-binding protein (MID) is a multimeric outer membrane protein belonging to the family of autotransporters. The IgD-binding domain of MID is located between amino acids MID 962-1200 and binds to amino acids 198-224 of the IgD C(H)1 region. In the present study, we describe a method to purify IgD from serum with high levels of IgD using a two-step affinity chromatography process. The first step involves depletion of MID-specific antibodies of all classes from serum using the non-IgD-binding fragment MID(1000-1200). This step is followed by selective capture of IgD with MID(962-1200). Furthermore, we demonstrate that the eluted IgD is pure, intact and functional for use in downstream applications. Our approach reduces the non-specificity commonly associated with lectin-based IgD purification regimes that rely on glycosylation of the IgD molecule.

The riddle of the dual expression of IgM and IgD

Signalling through the B cell antigen receptor (BCR) is required for peripheral B lymphocyte maturation, maintenance, activation and silencing. In mature B cells, the antigen receptor normally consists of two isotypes, membrane IgM and IgD (mIgM, mIgD). Although the signals initiated from both isotypes differ in kinetics and intensity, in vivo, the BCR of either isotype seems to be able to compensate for the loss of the other, reflected by the mild phenotypes of mice deficient for mIgM or mIgD. Thus, it is still unclear why mature B cells need expression of mIgD in addition to mIgM. In the current review we suggest that the view that IgD has a simply definable function centred around the basic signalling function should be replaced by the assumption that IgD fine tunes humoral responses, modulates B cell selection and homeostasis and thus shapes the B cell repertoire, defining IgD to be a key modulator of the humoral immune response.

A conserved tetranucleotide repeat is necessary for wild-type expression of the Moraxella catarrhalis UspA2 protein

The UspA2 protein has been shown to be directly involved in the serum-resistant phenotype of Moraxella catarrhalis. The predicted 5'-untranslated regions (UTR) of the uspA2 genes in several different M. catarrhalis strains were shown to contain various numbers (i.e., 6 to 23) of a heteropolymeric tetranucleotide (AGAT) repeat. Deletion of the AGAT repeats from the uspA2 genes in the serum-resistant M. catarrhalis strains O35E and O12E resulted in a drastic reduction in UspA2 protein expression and serum resistance. PCR and transformation were used to construct a series of M. catarrhalis O12E strains that differed only in the number of AGAT repeats in their uspA2 genes. Expression of UspA2 was maximal in the presence of 18 AGAT repeats, although serum resistance attained wild-type levels in the presence of as few as nine AGAT repeats. Increased UspA2 expression was correlated with both increased binding of vitronectin and decreased binding of polymerized C9. Real-time reverse transcription-PCR analysis showed that changes in the number of AGAT repeats affected the levels of uspA2 mRNA, with 15 to 18 AGAT repeats yielding maximal levels. Primer extension analysis indicated that these AGAT repeats were contained in the 5'-UTR of the uspA2 gene. The mRNA transcribed from a uspA2 gene containing 18 AGAT repeats was found to have a longer half-life than that transcribed from a uspA2 gene lacking AGAT repeats. These data confirm that the presence of the AGAT repeats in the 5'-UTR of the uspA2 gene is necessary for both normal expression of the UspA2 protein and serum resistance.

The IgD CH1 region contains the binding site for the human respiratory pathogenMoraxella catarrhalis IgD-binding protein MID

The Moraxella catarrhalis IgD-binding protein (MID) has a unique specificity for human IgD, and the sequence with maximal IgD binding is located within the amino acids MID962-1200. In the present paper, we examined the MID binding site on IgD using a series of recombinant Ig. Full-length IgD, IgD F(ab')2, and an IgD F(ab') C290R mutant lacking the inter-heavy-chain cysteine 290 were manufactured. Furthermore, a series of IgD/IgG chimeras were constructed. ELISA, dot blot and flow cytometry were used to study the binding of purified Ig to native MID, recombinant MID962-1200 or to Moraxella with or without MID. MID962-1200 bound both the IgD F(ab')2 and F(ab') C290R, indicating that the binding occurred independently of antibody structure. When amino acids 157-224 of the IgD CH1 region were substituted with IgG sequences, binding by M. catarrhalis or recombinant MID962-1200 was abolished. Subsequent smaller substitutions of IgD CH1 157-224 with IgG sequences led us to conclude that IgD CH1 amino acids 198-206 were crucial for the interaction between MID and IgD.

Biofilm formation by Moraxella catarrhalis in vitro: roles of the UspA1 adhesin and the Hag hemagglutinin

Mutant analysis was used to identify Moraxella catarrhalis gene products necessary for biofilm development in a crystal violet-based assay involving 24-well tissue culture plates. The wild-type M. catarrhalis strains that formed the most extensive biofilms in this system proved to be refractory to transposon mutagenesis, so an M. catarrhalis strain was constructed that was both able to form biofilms in vitro and amenable to transposon mutagenesis. Chromosomal DNA from the biofilm-positive strain O46E was used to transform the biofilm-negative strain O35E; transformants able to form biofilms were identified and subjected to transposon-mediated mutagenesis. Biofilm-negative mutants of these transformants were shown to have a transposon insertion in the uspA1 gene. Nucleotide sequence analysis revealed that the biofilm-positive transformant T14 contained a hybrid O46E-O35E uspA1 gene, with the N-terminal 155 amino acids being derived from the O46E UspA1 protein. Transformant T14 was also shown to be unable to express the Hag protein, which normally extends from the surface of the M. catarrhalis cell. Introduction of a wild-type O35E hag gene into T14 eliminated its ability to form a biofilm. When the hybrid O46E-O35E uspA1 gene from T14 was used to replace the uspA1 gene of O35E, this transformant strain did not form a biofilm. However, inactivation of the hag gene did allow biofilm formation by strain O35E expressing the hybrid O46E-O35E uspA1 gene product. The Hag protein was shown to have an inhibitory or negative effect on biofilm formation by these M. catarrhalis strains in the crystal violet-based assay.

Ionic binding of C3 to the human pathogen Moraxella catarrhalis is a unique mechanism for combating innate immunity

Moraxella catarrhalis ubiquitous surface proteins A1 and A2 (UspA1/A2) interfere with the classical pathway of the complement system by binding C4b-binding protein. In this study we demonstrate that M. catarrhalis UspA1 and A2 noncovalently and in a dose-dependent manner bind both the third component of complement (C3) from EDTA-treated serum and methylamine-treated C3. In contrast, related Moraxella subspecies (n = 13) or other human pathogenic bacteria (n = 13) do not bind C3 or methylamine-treated C3. Experiments with recombinant proteins and M. catarrhalis mutants devoid of UspA1/A2 revealed that UspA1/A2 exert their actions by absorbing and neutralizing C3 from serum and restrain complement activation. UspA2 was responsible for most of the effect, and the Moraxella mutant lacking UspA2 was more sensitive to the lytic effect of human serum compared with the wild type. Interestingly, among the large number of bacteria analyzed, only M. catarrhalis has this unique ability to interfere with the innate immune system of complement by binding C3.

The IgD-binding domain of the Moraxella IgD-binding protein MID (MID962-1200) activates human B cells in the presence of T cell cytokines

Moraxella catarrhalis immunoglobulin D (IgD)-binding protein (MID) is an outer membrane protein with specific affinity for soluble and cell-bound human IgD. Here, we demonstrate that mutated M. catarrhalis strains devoid of MID show a 75% decreased activation of human B cells as compared with wild-type bacteria. In contrast to MID-expressing Moraxella, the MID-deficient Moraxella mutants did not bind to human CD19+ IgD+ B cells. The smallest MID fragment with preserved IgD-binding capacity comprises 238 amino acids (MID(962-1200)). To prove the specificity of MID(962-1200) for IgD, a Chinese hamster ovary (CHO) cell line expressing membrane-anchored human IgD was manufactured. MID(962-1200) bound strongly to the recombinant IgD on CHO cells. Moreover, MID(962-1200) stimulated peripheral blood lymphocyte (PBL) proliferation 5- and 15-fold at 0.1 and 1.0 microg/ml, respectively. This activation could be blocked completely by antibodies directed against the CD40 ligand (CD154). MID(962-1200) also activated purified B cells in the presence of interleukin (IL)-2 or IL-4. An increased IL-6 production was seen after stimulation with MID(962-1200), as revealed by a human cytokine protein array. MID(962-1200) fused to green fluorescent protein (GFP) bound to human B cells and activated PBL to the same degree as MID(962-1200). Taken together, MID is the only IgD-binding protein in Moraxella. Furthermore, the novel T cell-independent antigen MID(962-1200) may, together with MID(962-1200)-GFP, be considered as promising reagents in the study of IgD-dependent B cell activation.

Alpha1-antitrypsin inhibits Moraxella catarrhalis MID protein-induced tonsillar B cell proliferation and IL-6 release

Alpha1-antitrypsin (AAT) is a major circulating and tissues inhibitor of serine proteinases implicated in the regulation of inflammation and host defence. There is now increasing evidence that AAT may also exhibit anti-inflammatory activities independent of its protease inhibitor function. This study was undertaken to investigate the effects of native (inhibitory) and polymerized (non-inhibitory) forms of AAT on MID (Moraxella IgD binding protein)-induced human tonsillar B cell activation in vitro. We found that 0.5 microg/ml MID induces B cell proliferation and stimulates IL-6 release (p<0.001) relative to non-stimulated controls. Both native and polymerized AAT (0.5 mg/ml) inhibited MID-stimulated B cell proliferation in a similar manner (by 70%, p<0.001), whereas MID-induced IL-6 release was more strongly suppressed by polymerized (9.9-fold, p<0.001) as compared to native AAT (2.8-fold, p<0.01). Electrophoretic analysis of cell culture media did not indicate any interaction between AAT and MID, and flow cytometry data showed no competition for the same receptor. The effects of AATs were observed whether added together with MID or 2h after MID-addition to cell cultures. Thus, our data demonstrate that AAT inhibits MID-induced B cell activation in vitro that is unrelated to its protease inhibitory activity and is not dependent on MID binding to the cell surface.

Hag directly mediates the adherence of Moraxella catarrhalis to human middle ear cells

Moraxella catarrhalis is a human pathogen that causes otitis media in young children and lung infections in patients with chronic obstructive pulmonary disease. In this study, the role of the surface protein Hag in the adherence of multiple M. catarrhalis strains was examined. The hag genes of four clinical isolates were disrupted with a spectinomycin resistance cassette, and the binding of isogenic mutants to primary cultures of human middle ear epithelial cells (HMEE), as well as A549 pneumocytes, was measured. These experiments revealed that the attachment of most mutants to both cell types was 10-fold less than that of their wild-type progenitors. To determine whether Hag directly mediates adherence to human cells, the hag genes from three M. catarrhalis isolates were cloned and expressed in a nonadherent Escherichia coli cloning strain. At least 17-fold more E. coli bacteria expressing Hag attached to HMEE cells than an adherence-negative control. Surprisingly, Hag expression did not increase the binding of recombinant E. coli to A549 monolayers. Our data demonstrate that the involvement of Hag in M. catarrhalis adherence to A549 and HMEE cells is conserved among isolates and that Hag directly mediates binding to HMEE cells.

The CD19 Molecule is Crucial for MID-Dependent Activation of Tonsillar B Cells from Children

The Moraxella immunoglobulin (Ig) D-binding protein (MID) induces a strong proliferative response in human peripheral blood IgD+ B cells from adults isolated by positive selection using anti-CD19-conjugated microbeads. Here, we show that tonsillar B cells from children isolated with positive selection are unable to respond to MID stimulation. The proliferative response was very low or absent at various concentrations of MID tested and at different time points analysed, whereas the MID response of tonsillar B cells from adults isolated with positive selection was considerably higher. Tonsillar B cells from children isolated with positive selection responded to formalin-fixed preparations of Moraxella catarrhalis and Staphylococcus aureus Cowan strain I. In comparison to cells isolated with positive selection, a much higher proliferative response was recorded in tonsillar B cells from children isolated with negative selection, indicating that occupation of the CD19 molecule (i.e. positive selection) inhibited the response. Indeed, the addition of anti-CD19 monoclonal antibodies (MoAb) to MID-activated tonsillar B cells from children isolated with negative selection strongly inhibited the proliferative response. In contrast, anti-CD21 MoAb at the same concentration did only show a minor inhibition on the MID-induced response. Pre-incubation of tonsillar B cells isolated from children with anti-CD19 or anti-CD21 MoAb did not affect the binding of biotin-conjugated MID as analysed by flow cytometry. These results suggest that MID-activated tonsillar B cells from children have a strong requirement for signalling through the CD19 molecule. Future experiments will further reveal the importance of CD19 and possibly other molecules for optimal activation of tonsillar B cells isolated from both children and adults.